1. Field of the Invention
The present invention relates generally to fluid reaction surfaces, and more specifically to a process for making a thin walled turbine blade.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Turbine airfoils, rotor blades and stator vanes, used in a gas turbine engine require internal cooling because of the extremely hot gas flow passing over the airfoil surface of these airfoils. Turbine airfoils have a rigid internal web or rib portion with a thin airfoil wall forming the airfoil surface on which the hot gas flow is exposed. Thin wall airfoils are used in the lower stages of the turbine that require longer airfoils, and therefore a more rigid internal structure to support the airfoil under the high stress levels during operation of the turbine. The internal ribs form the internal cooling passages and impingement cavities. Thin wall airfoils provide a high level of heat transfer from the hot external surface to the cooled interior surface of the wall.
A thin wall turbine blade made from a super alloy, such as a nickel based super alloy, cannot be cast by the conventional lost wax casting process. Wall thicknesses required for a thin wall turbine blade cannot be cast because the molten metal is not viscous enough to pass through all of the narrow cavities that form the thin walls. The prior art thin wall turbine blades are therefore made by other processes such as that disclosed in U.S. Pat. No. 6,805,535 B2 issued to Tiemann on Oct. 19, 2004 and entitled DEVICE AND METHOD FOR PRODUCING A BLADE FOR A TURBINE AND BLADE PRODUCED ACCORDING TO THIS METHOD in which the blade is cast as two halves, and then the two halves are bonded tog ether to form the finished thin wall blade.
Another process for making thin wall turbine blades is disclosed in the U.S. Pat. No. 5,640,767 issued to Jackson et al on Jun. 24, 1997 and entitled METHOD FOR MAKING A DOUBLE-WALL AIRFOIL which shows the blade made from a partially hollow airfoil support wall, and a thin wall airfoil shaped outer surface bonded over the support wall. This type of thin wall blade is a composite blade.
Another type of composite turbine blade is shown in U.S. Pat. No. 5,348,446 issued to Lee et al on Sep. 20, 1994 and entitled BIMETALLIC TURBINE AIRFOIL which shows the blade made from a core body with first and second panels bonded to the pressure and suction sides of the core body and fabricated leading and trailing edge components bonded to the leading and trailing edges of the core body to form the composite blade. All of these above cited prior art references disclose a process for making a blade in which the blade is formed of multiple parts and not cast as a single piece.
The current casting process to produce a turbine blade will produce wall thickness based on the casting alloy used and the grain structure desired. The single crystal casting process will produce a thin wall turbine blade. However, this process is very expensive to produce a turbine blade.
It is an object of the present invention to produce a turbine blade with a thin wall airfoil surface by casting the blade as a single piece without forming the blade from a plurality of parts that are bonded together.
Another object of the present invention is to produce a thin walled turbine blade that is much lower in cost than the single crystal cast turbine blade of the prior art.